1. Field of the Invention
This invention relates to the field of spectroscopy. More specifically, this invention relates to a fiber optic evanescent absorption sensor.
2. Description of the Prior Art
In conventional absorption spectroscopy, the accurate determination of the absorbance on a given band in the absorbing medium requires an optimum path length. Thus, a more strongly absorbing band requires a shorter path than a weaker one. Similarly, an absorption measuring apparatus with a fixed path length cannot be optimal for the measurement of absorption on a number of bands with widely different absorption coefficients.
A conventional fiber optic evanescent wave absorption sensor uses one source and one detection system and is optimized for measuring absorbance that varies in magnitude by a factor of approximately 30. On the other hand, for example, the absorbance in the mid-IR is typically 100 times or more larger than that in the near-IR, and therefore the conventional evanescent wave absorption sensor cannot be configured to be optimal for both regions.
What is needed is a device and method that permits a larger range of absorbance to be determined with high accuracy for a fixed interaction length.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the art could be overcome.